It's not a mystery that we'll ever solve, and even the Church refused to take sides with either the Thomists or Molinists.

Predestination follows from the notion that grace and salvation are a free and unmerited gift from God.  To say that it's given as a reward for activities of free will is to say that people effectively earn their salvation, and it's Pelagianism.  There's nothing that we can do that merits salvation.

It's a delicate balance between sufficient grace and efficacious grace.  Whereas a small amount of efficacious grace would suffice for the salvation of one, a tremendous amount of efficacious grace would be required for another, to the point that it would almost have to overwhelm the individual's free will.  Only God knows all the criteria that go into the amount of grace He gives to each individual, and we know that He is both all Merciful and all Just ... and that's really all we need to know.  St. Augustine actually tried to dissuade people from churning over questions about why some were saved and some were not, asserting that that kind of thinking leads to a vortex of confusion.  I don't give too much thought to it, but am content with knowing that God is all Merciful and all Just, and that my pea brain is not in any position to make any judgment regarding God's judgments.  How many people lose the faith over such considerations, presuming to sit in judgment of God!  Why did God allow this or that tragedy?  No Merciful God would allow such a thing.  Why did God allow some people to born among animists in the jungle but others were raised by saintly parents and became saints themselves?  That's unfair, so now we have to theorize about Baptism of Desire, etc. ... to make is so that God conforms to our judgments regarding would would be fair and what wouldn't.  St. Augustine admitted that this thinking was behind Baptism of Desire theory, and he rejected it, saying that "if you wish to remain Catholic," you'll avoid such speculations.

God is perfectly Merciful and perfectly Just ... we know this with the certainty of faith.  Our pea brains are not capable of sitting in judgment of Him.  End of story.

Romans, Chapter 8:29-30

http://drbo.org/cgi-bin/d?b=drb&bk=52&ch=8&l=29-#x

[29] For whom he foreknew, he also predestinated to be made conformable to the image of his Son; that he might be the firstborn amongst many brethren. [30] And whom he predestinated, them he also called. And whom he called, them he also justified. And whom he justified, them he also glorified.

Haydock Commentary:

https://www.ecatholic2000.com/haydock/ntcomment132.shtml

Ver. 29. For whom he foreknew, he also predestinated to be made conformable to the image of his Son, in suffering with Christ, in following his doctrine, in imitating his life. This foreknowledge of God, according to S. Augustin,[6] is not merely a foreseeing of what men will do by the assistance and graces of God's ordinary providence, much less a foreseeing of what they will do by their own natural strength, as the Pelagian heretics pretended: but is a foreknowledge including an act of the divine will, and of his love towards his elect servants; (as to know in the Scriptures, when applied to God, is many times the same as to approve and love) God therefore hath foreseen or predestinated, or decreed that these elect, by the help of his special graces, and by the co-operation of their free-will, should be conformable to the image of his Son, that so his Son, even as man, might be the first-born, the chief, and the head of all that shall be saved. Wi. — God hath preordained that all his elect shall be conformable to the image of his Son. We must not here offer to dive into the secrets of God's eternal election: only firmly believe that all our good, in time and eternity, flows originally from God's free goodness; and all our evil from man's free will. Ch.

Council of Trent: On Justification

http://www.thecounciloftrent.com/ch6.htm



[. . .]

CHAPTER XII.

 That a rash presumptuousness in the matter of Predestination is to be avoided.
 
No one, moreover, so long as he is in this mortal life, ought so far to presume as regards the secret mystery of divine predestination, as to determine for certain that he is assuredly in the number of the predestinate; as if it were true, that he that is justified, either cannot sin any more, or, if he do sin, that he ought to promise himself an assured repentance; for except by special revelation, it cannot be known whom God hath chosen unto Himself.

[. . .]


CANONS


[. . .]


CANON XV.-If any one saith, that a man, who is born again and justified, is bound of faith to believe that he is assuredly in the number of the predestinate; let him be anathema.

CANON XVI.-If any one saith, that he will for certain, of an absolute and infallible certainty, have that great gift of perseverance unto the end,-unless he have learned this by special revelation; let him be anathema.

CANON XVII.-If any one saith, that the grace of Justification is only attained to by those who are predestined unto life; but that all others who are called, are called indeed, but receive not grace, as being, by the divine power, predestined unto evil; let him be anathema.

St. Augustine, On Rebuke and Grace


https://www.newadvent.org/fathers/1513.htm

Chapter 23.— Those Who are Called According to the Purpose Alone are Predestinated.

[. . .]


Whosoever, therefore, in God's most providential ordering, are foreknown, predestinated, called, justified, glorified — I say not, even although not yet born again, but even although not yet born at all, are already children of God, and absolutely cannot perish. These truly come to Christ, because they come in such wise as He Himself says, "All that the Father gives me shall come to me, and him that comes to me I will not cast out;" John 6:37 and a little after He says, "This is the will of the Father who has sent me, that of all that He has given me I shall lose nothing." John 6:39 From Him, therefore, is given also perseverance in good even to the end; for it is not given save to those who shall not perish, since they who do not persevere shall perish.

Chapter 40.— No One is Certain and Secure of His Own Predestination and Salvation.

But, moreover, that such things as these are so spoken to saints who will persevere, as if it were reckoned uncertain whether they will persevere, is a reason that they ought not otherwise to hear these things, since it is well for them not to be high-minded, but to fear. Romans 11:20 For who of the multitude of believers can presume, so long as he is living in this mortal state, that he is in the number of the predestinated? Because it is necessary that in this condition that should be kept hidden; since here we have to beware so much of pride, that even so great an apostle was buffetted by a messenger of Satan, lest he should be lifted up.

Chapter 42.— The Reprobate are to Be Punished for Merits of a Different Kind.

But those who do not belong to this number of the predestinated, whom — whether that they have not yet any free choice of their will, or with a choice of will truly free, because freed by grace itself — the grace of God brings to His kingdom, — those, then, who do not belong to that most certain and blessed number, are most righteously judged according to their deservings. For either they lie under the sin which they have inherited by original generation, and depart hence with that inherited debt which is not put away by regeneration, or by their free will have added other sins besides; their will, I say, free, but not freed — free from righteousness, but enslaved to sin, by which they are tossed about by various mischievous lusts, some more evil, some less, but all evil; and they must be adjudged to diverse punishments, according to that very diversity. Or they receive the grace of God, but they are only for a season, and do not persevere; they forsake and are forsaken. For by their free will, as they have not received the gift of perseverance, they are sent away by the righteous and hidden judgment of God.

Chapter 44.— In What Way God Wills All Men to Be Saved.

And what is written, that He wills all men to be saved, 1 Timothy 2:4 while yet all men are not saved, may be understood in many ways, some of which I have mentioned in other writings of mine; but here I will say one thing: He wills all men to be saved, is so said that all the predestinated may be understood by it, because every kind of men is among them. Just as it was said to the Pharisees, You tithe every herb; Luke 11:42 where the expression is only to be understood of every herb that they had, for they did not tithe every herb which was found throughout the whole earth. According to the same manner of speaking, it was said, Even as I also please all men in all things. 1 Corinthians 10:33 For did he who said this please also the multitude of his persecutors? But he pleased every kind of men that assembled in the Church of Christ, whether they were already established therein, or were to be introduced into it.

Mystici Corporis

The Mystical Body of Christ, the Church

Pope Pius XII - 1943



https://www.papalencyclicals.net/pius12/p12mysti.htm

23. Nor must one imagine that the Body of the Church, just because it bears the name of Christ, is made up during the days of its earthly pilgrimage only of members conspicuous for their holiness, or that it consists only of those whom God has predestined to eternal happiness.

St. Thomas Aquinas

DE RATIONIBUS FIDEI

REASONS FOR THE FAITH, AGAINST MUSLIM OBJECTIONS



https://isidore.co/aquinas/english/Rationes.htm#10

Chapter 10: That divine predestination does not impose necessity on human acts

Last of all we come to the question whether, because of divine ordination or predestination, human acts become necessary. This question requires caution so as to defend the truth and avoid falsity or error.

It is erroneous to say that human acts and events escape God's fore-knowledge and ordination. It is no less erroneous to say that God's fore-knowledge and ordination imposes necessity on human acts; otherwise free will would be removed, as well as the value of taking counsel, the usefulness of laws, the care to do what is right and the justice of rewards and punishments.

We must observe that God knows things differently from man. Man is subject to time and therefore knows things temporally, seeing some things as present, recalling others as past, and fore-seeing others as future. But God is above the passage of time, and his existence is eternal. So his knowledge is not temporal, but eternal. Eternity is compared to time as something indivisible to what is continuous. Thus in time there is a difference of successive parts according to before and after, but eternity has no before and after, because eternal things are free from any change.

Thus eternity is totally at once, just as a point lacks parts that are distinct in location. For a point can be compared to a line in two ways: first as included in the line, whether at the beginning, middle or end, secondly as existing outside a line. A point within a line cannot be present to all the parts of the line, but in different parts of the line different points must be designated. But a point outside the line can view all parts of the line equally, as in a circle, whose central point is indivisible and faces all the parts of the circuмference and all of them are somehow present to it, although not to one another.

An instant, which is a limit of time, is comparable to the point included in a line. It is not present to all parts of time, but in different parts of time different instances are designated. Eternity is something like the point outside a line, like the centre of a circle. Since it is simple and indivisible, it comprehends the whole passage of time and each part of time is equally present to it, although one part of time follows another.

Thus God, who looks at everything from the high point of eternity, views as present the whole passage of time and everything that is done in time. Therefore, when I see Socrates sitting, my knowledge is infallible and certain, but no necessity is imposed on Socrates to be seated. Thus God, seeing everything that is past, future or present to us as present to himself, knows all this infallibly and certainly, yet without imposing on contingent things any necessity of existing.

This comparison can be accepted, if we compare the passage of time to travel over a road. If someone is on a road over which many people pass, he sees those who are just ahead of him, but cannot certainly know those who come after him. But if someone stands in a high place where he can see the whole road, he sees at once all who are moving on the road. Thus man, who is in time, cannot see the whole course of time at once, but only thinks that just in front of him, namely the present, and a few things of the past, but he cannot know future things for certain. But God, from the high point of his eternity sees with certitude and as present all that is done through the whole course of time, without imposing necessity on contingent things.

Just as God's knowledge does not impose necessity on contingent things, neither does his ordination, by which he providentially orders the universe. For he orders things the way he acts on things; his ordination does not violate but brings to effect by his power what he planned in his Wisdom.

As for the action of God's power, we should observe that he acts in everything and moves each single thing to its actions according to the manner proper to each thing, so that some things, by divine motion, act from necessity, as the motion of heavenly bodies [according to ancient cosmology], while others contingently, which sometimes fail in their proper action because of their corruptibility. A tree, for example, sometimes is impeded from producing fruit and an animal from generating offspring. Thus Divine Wisdom orders things so that they happen after the manner of their proper causes. In the case of man, it is natural for him to act freely, not forced, because rational powers can turn in opposite directions. Thus God orders human actions in a way that these actions are not subject to necessity, but come from free will.

St. John of Damascus

An Exposition of the Orthodox Faith

Book II


https://www.newadvent.org/fathers/33042.htm

Chapter 30. Concerning Prescience and Predestination.


We ought to understand that while God knows all things beforehand, yet He does not predetermine all things. For He knows beforehand those things that are in our power, but He does not predetermine them. For it is not His will that there should be wickedness nor does He choose to compel virtue. So that predetermination is the work of the divine command based on fore-knowledge. But on the other hand God predetermines those things which are not within our power in accordance with His prescience. For already God in His prescience has prejudged all things in accordance with His goodness and justice.

Bear in mind, too , that virtue is a gift from God implanted in our nature, and that He Himself is the source and cause of all good, and without His co-operation and help we cannot will or do any good thing. But we have it in our power either to abide in virtue and follow God, Who calls us into ways of virtue, or to stray from paths of virtue, which is to dwell in wickedness, and to follow the devil who summons but cannot compel us.

St. Thomas Aquinas

Summa Theologiae, First Part


https://www.newadvent.org/summa/1023.htm

Question 23. Predestination

• Is predestination suitably attributed to God?
• What is predestination, and does it place anything in the predestined?
• Does the reprobation of some men belong to God?
• The comparison of predestination to election; that is, are the predestined chosen?
• Are merits the cause or reason of predestination, or reprobation, or election?
• The certainty of predestination: will the predestined infallibly be saved?
• Is the number of the predestined certain?
• Can predestination be furthered by the prayers of the saints?

St. Thomas Aquinas

Questiones Disputatae de Veritate

TRUTH


https://isidore.co/aquinas/english/QDdeVer6.htm



Question Six: Predestination

1.  2.  3.  4.  5.  6.

Does predestination belong to knowledge of will?  Is foreknowledge of merits the cause of or reason for predestination?  Is predestination certain?  Is the number of predestined certain?  Are the predestined certain of their predestination?  Can predestination be helped by the prayers of the saints?

St. Thomas Aquinas


CONTRA GENTILES

BOOK THREE: PROVIDENCE

Part II: Chapters 84-163




https://isidore.co/aquinas/english/ContraGentiles3b.htm#162



Chapter 162

THAT GOD IS NOT THE CAUSE OF SIN FOR ANY PERSON

[1] Although God does not convert certain sinners to Himself, but leaves them in their sins according to their merits, He does not lead them into sinful action.

[2] In fact, men sin because they turn away from Him Who is their ultimate end, as is evident from our earlier statements.But, when every agent acts for an end that is proper and suitable to it, it is impossible by the action of God for any of them to be turned away from the ultimate end, Who is God. So, it is impossible for God to cause any persons to sin.

[3] Again, good cannot be the cause of evil. But sin is an evil for man, since it is opposed to man’s proper good which is to live in accord with reason. Therefore, it is impossible for God to be the cause of sinful action for anyone.

[4] Besides, all wisdom and goodness in man are derived from the wisdom and goodness of God, as a certain likeness of Him. But it is incompatible with human wisdom and goodness to cause anyone to sin; much more, then, is it incompatible with these divine qualities.

[5] Moreover, every sin stems from a defect in the proximate agent, and not from the influence of the primary agent: as the defect of limping results from the condition of the leg bone and not from the motor power, for, in fact, whatever perfection of motion is apparent in the act of limping, it is due to this power. But the proximate agent of human sin is the will. Therefore, the defect of sin comes from the will of man and not from God Who is the primary agent; from Him, however, comes whatever pertains to perfection of action in the sinful act.

[6] Hence, it is said in Sirach (15:12): “Say not: He caused me to err. For He has no need of wicked men.” And later: “He commanded no man to act wickedly, and He has given no man license to sin” (Sirach 15:,21). And in James (1:13) it is said: “Let no man, when he is tempted, say that he is tempted by God: for God is not a tempter of evils.”

[7] However, some passages are found in Scripture, from which it seems that God is the cause of sinning for certain men. Indeed, it is said in Exodus (10:1): “I have hardened Pharaoh’s heart, and the heart of his servants”; and in Isaiah (6:10): “Blind the heart of this people, and make their ears heavy... lest they see with their eyes... and be converted, and I heal them”; and in Isaiah (63:17): “You made us err from Your ways; You have hardened our heart, lest we fear You.” Again, in Romans (1:28) it is said: “God delivered them up to a reprobate sense, to do those things which are not convenient.” All these texts are to be understood in this way: God does not grant to some people His help in avoiding sin, while to others He does grant it.

[8] Moreover, this help is not only the infusing of grace, but also external guardianship, whereby the occasions of sinning are taken away from man by divine providence and whereby provocations to sin are suppressed. God also helps man in opposing sin by the natural light of reason and by the other natural goods which He accords man. So, when He takes away these aids from some, according to the merit of their action, as His justice demands, He is said to harden or to blind them, or to do any of the other things mentioned.

Chapter 163

ON PREDESTINATION, REPROBATION, AND DIVINE ELECTION

[1] So, since we have shownthat some men are directed by divine working to their ultimate end as aided by grace, while others who are deprived of the same help of grace fall short of their ultimate end, and since all things that are done by God are foreseen and ordered from eternity by His wisdom, as we showed above, the aforementioned differentiation of men must be ordered by God from eternity. According, then, as He has preordained some men from eternity, so that they are directed to their ultimate end, He is said to have predestined them. Hence, the Apostle says, in Ephesians (1:5): “Who predestinated us unto the adoption of children... according to the purpose of His will.” On the other hand, those to whom He has decided from eternity not to give His grace He is said to have reprobated or to have hated, in accord with what we find in Malachi (1:2-3): “I have loved Jacob, but have hated Esau.” By reason of this distinction, according to which He has reprobated some and predestined others, we take note of divine election, which is mentioned in Ephesians (1:4): “He chose us in Him, before the foundation of the world.”

[2] Thus, it appears that predestination, election, and reprobation constitute a certain section of divine providence, according as men are ordered to their ultimate end by divine providence. Hence, it is possible to show that predestination and election impose no necessity, by the same reasoning whereby we showed above that divine providence does not take away contingency from things.

[3] Moreover, that predestination and election do not find their cause in any human merits can be made clear, not only from the fact that God’s grace which is the effect of predestination is not preceded by merits but rather precedes all human merits, as we showed, but it can also be shown from this, that the divine will and providence is the first cause of things that are done, but that there can be no cause of the divine will and providence, although, among the effects of providence, and likewise of predestination, one may be the cause of another.

“For who,” as the Apostle says (Rom. 11:35-36), “has first given to Him, and who shall make recompense to Him? For of Him, and in Him, and by Him, are all things. To Him be honor and glory for ever. Amen.”

Now that I was able to evoke a good discussion that explains to lurkers and members, who lack an understanding of Catholic theology on predestination that contrasts heretical protestant notions, I thank Ladislaus for the dandy contributions. Also, excellent call by Ladislaus for seeing the heretical teaching in the newadvent link regarding those souls outside of the Catholic Church who are "saved" because they are "predestined" like Catholics who die in the state of grace. 

Think closely on election in "for all" in the NO and the inherent contradiction with Matthew 22:14. "For many" in the True Mass is a statement about election, you should medidate on reconciling that with God being all-knowing.

I am trying to compress this into a manner that you will find attractive to give assent to an essential dimension of the Church's theology.

See my comment above yours.